1. Field of the invention
The present invention relates to a bidirectional light transmission system between transmitting and receiving terminals connected to a single optical fiber cable, and more particularly to a bidirectional light transmission system effective to optical CATV or the like and an optical device therefor.
2. Description of the prior art
As shown in FIGS. 1A to 1C, a conventional bidirectional light transmission system has used the multiplex technique. FIG. 1A illustrates an example of a space division wavelength multiplex technique as the multiplex technique. Reference numerals 2-1, 2-2, 4-1, 4-2 and 5 are a light source of a wavelength .lambda.1, a light source of a wavelength .lambda.2, light receivers and a transmission line respectively. FIG. 1B illustrates an example to which the wavelength multiplex technique is applied. In FIG. 1B, reference numerals 11 and 12 are an optical multiplexer and an optical demultiplexer respectively, and there is a single transmission line. FIG. 1C is an example to which a time division multiplex technique is applied. Reference numerals 13-1, 13-2 and 13-3 are terminals, reference numeral 13-4 is a center terminal, and reference numerals 13-5, 13-6 and 13-7 are nodes. The multiplex systems illustrated in FIGS. 1A to 1C are used alone or in combination with another. In particular, the time division multiplex technique has been widely used to an optical LAN (local area network) or the like.
Recently, a bidirectional light transmission system is used, in which a light source is not necessary for a receiving station because of the use of an electrooptic modulator. For example, E. J. MURPHY et al. has reported a bidirectional light transmission system as shown in FIG. 1D, in which a reflective external modulator 14 with a total reflecting mirror 15 at one end thereof is provided, and hence a light source is not necessary at a light receiving point of the system ("Simultaneous Single-Fiber Transmission of Video and Bidirectional Voice/Data Using LiNbO.sub.3 Guided-Wave Devices", JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 6, No. 6, pp. 937-945, June 1988). In FIG. 1D, reference numerals 1, 6 and 3-1 are a transmitting station, a receiving station and a light branching portion. Reference numerals 9 and 10 are a return signal and a transmission signal, respectively. J. K. WHEELER et al. has reported a combination system of two optical splitters, 3-2 and 3-3 and an optical phase modulator 7 as shown in FIG. 1E ("Two-way Transmission using Electro-Optical Modulator" ELECTRONICS LETTERS, 24th, April 1986, Vol. 22, No. 9 pp.479-481). In these two systems shown in FIGS. 1D and 1E, a video signal with a high bit rate is transmitted from a transmitting station and a sound signal or a data signal with a low bit rate is transmitted from a receiving station.
The conventional transmission systems shown in FIGS. 1A to 1C require a plurality of light sources and have a problem on their construction, reliability and economy.
The conventional transmission system shown in FIG. 1D has an advantage that a light source is not required at a receiving point, but has a disadvantage that a transmitted signal is disturbed by an external modulator 14. Consequently, a measure is necessary for removing the disturbance at a transmitting station or a receiving station.
In the conventional transmission system shown in FIG. 1E, two light branching devices 3-2 and 3-3 are used. Since, however, optical loss of at least 3 dB or more is generated, a level of received light is markedly lowered. Moreover, since reflected light passes through the light branching device again, far more optical loss is generated. An amount of reflected light is determined by the number of light branching device, and hence flexible design suitable for the system can not be made. Moreover, the system shown in FIG. 1E has a disadvantage that a modulation ratio is limited by the difference between the optical path lengths of two reflected light beams or the like.
In the bidirectional light transmission system, various optical devices such as an optical modulator and an optical switch are used.
The conventional optical modulator is a device of a transmission type and dependent on polarization of light. In general, an LiNbO.sub.3 crystal is used for a substrate, and a waveguide is formed on a Z-cut LiNbO.sub.3 Light is transmitted in the X axis direction, and an electric field is applied in the Z axis direction.
FIG. 2A illustrates an example of such a conventional optical modulator (see Nishihara, Haruna and Suhara, OPTICAL INTEGRATED CIRCUIT, pp.298-309, Ohmu Co., Ltd, 1985). In FIG. 2A, reference numerals 21, 22, 23 and 26 are a crystal substrate, an optical waveguide, a Y shape branched waveguide or a directional coupling waveguide, and an electrode respectively. Since constructed as described above, the conventional optical modulator shown in FIG. 2A has different electrooptic coefficients: an electrooptic coefficient in a TE mode and an electrooptic coefficient in a TM mode. Generally, light in a TM mode can be only modulated because the electrooptic coefficient can be made large.
Moreover, as shown in FIG. 2B, a reflective optical modulator having a light reflecting portion 24 is used. Reference numeral 27 is an asymmetrical X branched waveguide. The optical modulator also depends on polarization of light and has a total reflection type reflector (see Nishihara, "Recent Progress of Optical Integrated Circuit Devices", PAPERS OF TECHNICAL GROUP ON OPTICS AND OUANTUM ELECTRONICS OF I.E.I.C.E JAPAN, Vol. OQE86, No.123, pp. 47-54).
The conventional optical modulator shown in FIG. 2A depends on polarization of light. If a specific optical fiber for maintaining a plane of polarization is not used, the optical modulator can be only used just after a light source (semiconductor laser or the like). Furthermore, the conventional optical modulator shown in FIG. 2B depends on polarization of light, has a total reflection type reflector and the use thereof is limited to a special field of a thermal sensor, a pressure sensor or the like.
On the other hand, there are few examples of a reflective optical switch having a light reflecting portion, and there is only a device having a wavelength filter at its light reflecting portion (see E. J. MURPHY et al. "Simultaneous Single-Fiber Transmission of Video Bidirectional Voice/Data Using LiNbO.sub.3 Guided-Wave Devices", JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 6, No. 6 June 1988) . FIG. 2C illustrates the above conventional optical switch. In the optical switch, there are provided optical waveguides 22 and 22' on a Z-cut LiNbO.sub.3 substrate 21, an electrode 26 for applying an electric field in the Z axis direction of the crystal and a light reflecting portion 24. The optical waveguides 22 and 22' approach each other to construct an optical directional coupler and turn on or off a light input to a light receiving portion by presence or absence of a voltage applied to the electrode 26. This means that the optical device functions as an optical switch independent of polarization of light.
When an optical switching operation independent of polarization of light is realized in an optical directional coupler, for realizing a complete cross state is, it is necessary to match three parameters with high accuracy, i.e., a shape(phase constant), intensity of coupling and a coupling length of an optical waveguide (phase constant ) . Thus, severe manufacturing requirements and high fabricating accuracy are needed. Additionally, switching characteristics vary depending on a light wavelength or a difference in temperatures. The above directional coupling optical switch independent of polarization of light has such a disadvantage that the manufacturing requirements are rigidly restricted and a high operation voltage of 30 V to 70 V is necessary.
An optical switch shown in FIG. 2C is also usable for an optical modulator. Since, however, this device is a total reflecting type and a light receiver 25 thereof receives a switching light output, signal light superposed on incident light is disturbed by a modulation signal of the optical modulator. Moreover, in some cases, the received light disappears. Since a substrate is a Z-cut LiNbO.sub.3 crystal, light is affected by birefringence, and hence analog modulation thereof is hardly made, characteristics of a modulation factor and an extinction ratio are poor and signal light may be easily affected by polarization of light and manufacturing of the device is very difficult, and the like.